1. Field of the Invention
The field of the invention relates generally to preparing III-V compound films.
2. Background
It is well known that III-V nitrides (InN, GaN, AlN, and their alloys) can be used for semiconducting applications such as Light Emitting Diodes (xe2x80x9cLED""sxe2x80x9d) and Laser Diodes (xe2x80x9cLD""sxe2x80x9d) in the blue to yellow range of the visible spectrum, UV photodetectors, and many other applications. These III-V nitrides are attractive because of their tunable bandgaps (1.9 eV, InN to 3.4 eV, GaN to 6.2 eV, AlN) and because of their chemical stabilities, resistance to radiation, and high thermal conductivity""s. Neumayer et al.
Unfortunately, gallium nitride (xe2x80x9cGaNxe2x80x9d) has a large equilibrium dissociation pressure of N2 at its melting temperature and vapor growth temperatures. Thus, single crystals of GaN cannot be grown from the melt (liquid phase epitaxy, xe2x80x9cLPExe2x80x9d). Instead, GaN single crystals are grown heteroepitaxially on single crystal substrates acting as heterogeneous nucleation sites.
Growth methods of highly crystalline, single crystal III-V materials, specifically GaN, focus on vapor transport processes. Such vapor processes include molecular beam epitaxy (xe2x80x9cMBExe2x80x9d) and metal organic chemical vapor deposition (xe2x80x9cMOCVDxe2x80x9d). Both processes involve the transport of gaseous species (atomic, ionic or molecular) to a heated single crystal substrate. The molecular or ionic species either crack and then adsorb onto the substrate surface or crack after they are adsorbed on the surface. After the species are adsorbed on the surface of the substrate, the heated substrate provides enough energy for their surface mobility. The species combine with the other element(s) and begin to form a layer of material. Depending on the surface energies of the film, interface and substrate, the material either grows layer by layer (Frank-van der Merwe), island coalescence (Volmer-Weber), or a modification of the two (Stranski-Krastanov). This growth of a single crystal material upon another is known as epitaxy.
Each vapor process has its limitations. The MOCVD process uses large quantities of ammonia in an attempt to keep a 1:1 ratio of group III element to nitrogen. Large quantities are necessary mainly due to (i) the large overpressure of nitrogen required to keep the GaN stable at high temperatures, and (ii) the poor cracking efficiency of ammonia into atomic nitrogen. Although the MBE process avoids the use of ammonia by producing atomic nitrogen from a RF plasma source, the process requires ultra high vacuum systems. Large quantities of ammonia, and ultra high vacuum levels, raise concerns about chemical safety and chemical disposal. Moreover, both MOCVD and MBD require large initial capital costs for equipment and maintenance.
A solution precursor route to form GaN thin films has been described in which GaN films are grown from a solution of gallium bis-(trimethylsilyl) carbodiimide. However, this precursor contains silicon and a strong nitrogen-carbon bond, and silicon and carbon impurities are left behind after pyrolysis and subsequent heat treatments. Rodewald et al. Silicon impurities in GaN have been known to act as unintentional shallow n-type dopants (increasing the electron carrier concentration) similar to oxygen impurities. Van de Walle et al. The silicon atom substitutes on the gallium site and has a low formation energy ( less than 1 eV) such that it can be readily incorporated into GaN. This has a negative effect when trying to produce LD""s, since p-type GaN can not be grown using this method. Carbon impurities are theoretically expected to act as deep acceptor states. However, experiments have shown that carbon impurities act as shallow donors in GaN. Fiorentini et al.; Leroux et al. The silicon and carbon impurities cause a broad yellow emission in the photoluminescence (xe2x80x9cPLxe2x80x9d) spectrum. Thus, the carbodiimide precursor is unsuitable for its intended electronic device applications.
For the foregoing reasons, there is a need for a process of producing III-V compound films that uses smaller volumes of ammonia and lower pressure levels, and that gives films without silicon or carbon impurities.
The present invention is directed to a process that satisfies the needs for smaller volumes of ammonia, lower pressure levels, and insignificant levels of silicon and carbon impurities. The new process for producing crystalline III-V compound films on crystal substrates comprises depositing an amorphous layer of a III-V compound precursor on a crystal substrate, reacting the amorphous layer with a reduced form of a Group V element, and then heating the amorphous layer at a temperature and for a time sufficient to crystallize the amorphous layer by pyrolysis. Preferably, the III-V compound precursor is deposited by coating a single crystal substrate with a solution of the precursor. Moreover, in the presence of ammonia and using suitable oxygen-containing precursors, the reacting and heating steps can be carried out simultaneously.
According to one version of the invention, III-V thin films are produced on single crystal substrates using the II-V compound precursor exemplified by gallium dimethyl amide (Ga2[N(CH3)2]6; xe2x80x9cGDAxe2x80x9d). This precursor contains gallium-nitrogen bonds which minimize the required amount of ammonia. An amorphous layer of the precursor is deposited by spin coating or dip coating a solution of GDA on a single crystal substrate, the amorphous layer is reacted with ammonia at room temperature to remove carbon groups from the precursor, and then the amorphous layer is heated in either nitrogen or ammonia to crystallize the amorphous layer by pyrolysis. The single crystal substrate, which has a similar crystalline structure to GaN, acts as a nucleation site. The film grows into a single crystal GaN layer at higher temperatures. This version on the invention is both simple and inexpensive (both capital and operating costs), and is carried out at atmospheric conditions, negating the need for costly high vacuum systems. Films of other III-V nitrides can be produced using the corresponding Group III-dimethyl amide.
According to another version of the invention, GaN thin films are produced on single crystal substrates using oxygen-containing, III-V compound precursors exemplified by either a gallium salt, gallium nitrate [Ga(NO3)3] (xe2x80x9cGNOxe2x80x9d), or an alkoxide of gallium, gallium isopropoxide [Ga(OC3H7)3] (xe2x80x9cGIPxe2x80x9d). These precursors are oxygen-containing molecules that are stable in air. An amorphous layer is deposited on a single crystal substrate by spin or dip coating a solution of GNO or GIP on a single crystal substrate, and the amorphous oxide layer is heated in ammonia to nitride the oxide layer and to remove carbon. The heating step also crystallizes the amorphous layer. This version of the invention is carried out under atmospheric conditions, negating the need for costly high vacuum systems. Other III-V nitride films can be produced using the corresponding Group III salt or alkoxide.
The operating frequency of devices made from films produced by this invention can be changed since aluminum and indium may be easily added to the precursor solution in exact amounts while p-dopants such as magnesium or n-dopants such as silicon may be added. These films can also be used as free standing entities or as buffer layers for CVD overgrowth in order to lower defect densities and improve overall efficiency of LED and LD devices. A significant advantage of this invention is its dramatic cost effectiveness versus conventional vapor processes.
The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.